Following the showing of the theatrical trailer next Monday, it appears that tickets will become available that day. Major theater chains such as AMC and Regal Cinemas have already started to prepare their systems in anticipation of ticket purchases. While there has been no confirmation from either Lucasfilm or Disney, this would make sense given the fact that the release date is only two and a half months away (December 15) and the theatrical trailer will be aired that day on Monday Night Football.
There is a precedent for this as well. After The Force Awakens theatrical trailer, tickets were immediately afterward. AMC sold out of tickets within 12 hours and services like Fandango and MovieTickets.com had an enormous surge of both traffic and sales following the trailer for The Force Awakens.
I simply bought my ticket the night of and saw a 7:00pm showing the night before the official release, but, it may not be so easy this time around. Everyone should plan ahead of time to make sure they snag their tickets for the most anticipated film of the year as soon as they can!